Method and device for growing large-volume oriented monocrystals

ABSTRACT

In the method for growing large-volume monocrystals crystal raw material is heated in a melting vessel with heating elements to a temperature above its melting point until a melt is formed. A monocrystal is then formed on the bottom of the melting vessel by lowering the temperature at least to the crystallization point. A solid/liquid phase boundary is formed between the monocrystal and the melt. The monocrystal grows towards the melt surface in a direction that is perpendicular to the phase boundary. A vertical axial temperature gradient is produced and maintained between the bottom of the melting vessel and its upper opening and heat inflow and/or heat outflow through side walls of the melting vessel is prevented, so that the solid/liquid phase boundary has a curvature radius of at least one meter. A crystal-growing device for performing this process is also described.

BACKGROUND OF THE INVENTION

The invention relates to a method for growing large-volume monocrystalsof uniform orientation from a melt, to a device for carrying out thismethod and to the use of crystals prepared in this manner.

Monocrystals are characterized by the fact that they have a uniformorientation throughout their entire volume which is a prerequisite forhigh optical homogeneity within the entire crystal volume. For thisreason, they are eminently suited for use in the optical industry or asstarting material for optical components in deep-ultraviolet [DUV]photolithography, for example for steppers or excimer lasers.

The growing of monocrystals from a melt is in itself known. Text booksabout crystal growing, for example “Kristallzüjchtung” [The Growing ofCrystals] by K. Th. Wilke and J. Bohm, which has 1088 pages, describe awide variety of different methods for crystal growing of which the mostcommon techniques will be mentioned briefly in the following. Inprinciple, crystals can be grown from the gas phase, from the melt, fromsolutions or even from a solid phase by recrystallization or diffusionthrough a solid body. Such methods, however, are meant primarily forlaboratory-scale work and not for large-scale industrial production. Themost important large-scale melt growing processes for making crystalswill be explained briefly in the following.

The Czochralski method involves dipping, with the aid of a finger-typetool, a slightly cooled seed crystal into a crucible containing moltencrystal raw material and then pulling this seed crystal out slowly,preferably with rotation. In this manner, during the pulling, the seedcrystal grows into a larger crystal.

The drawback of this method is that cooling produces relatively largetemperature changes in the crystal resulting in stress-inducedanisotropy.

By the vertical Bridgeman method, a crystal raw material is melted in amobile melting crucible by means of a heating jacket. The crucible isthen slowly lowered from the heating jacket through an axial temperaturegradient produced by heaters. Or, alternatively, the crucible isstationary, and a mobile heating system is moved upward. The melt isthus cooled allowing the slow growth of an added seed crystal. In avariant of this method known as the Bridgeman-Stockbager method, acrystal is formed by slowly lowering the mobile crucible in an axialgradient between two heating jackets disposed above each other andbetween which exists a major temperature difference.

In the vertical gradient freeze method (VGF method), several concentricheating coils are disposed over each other around the stationary meltingcrucible so as to form a jacket. Each of these coils can be controlledseparately. By slowly decreasing the heat output of each individualheating coil disposed around the crucible wall, the temperature isslowly reduced to below the crystallization point thus generating aradial temperature gradient along which crystal growth takes place.

In the gradient solidification method (GSM), a ring-shaped heating coilsurrounding a stationary melting crucible is slowly moved downward andthen upward.

Nevertheless, oriented monocrystals usually do not exhibit homogeneousoptical and mechanical properties. It is desirable that such crystals beproduced with a crystal orientation appropriate for a particularapplication. This, however, creates major problems in the production oflarge monocrystals, because during their growth such crystalsspontaneously change their orientation, namely the position of theircrystal axis. This leads to optically nonuniform crystals which do notexhibit the same light refraction in all regions.

Until now, while it was possible to produce crystals exhibiting some ofthese properties, it was not possible to grow large-volume uniformlyoriented crystals that are free of convergences, are optically highlyhomogenous, exhibit high transmission and, in addition, do not discolorwhen exposed to a strong radiation source.

Attempts have been made by hitherto known methods, for example in theproduction of large calcium fluoride monocrystals, to grow the crystalin the direction of the {111} axis. This gave very low yields, however,namely only about 6-8% of the growing attempts gave a satisfactorycrystal size. Because such crystal growing methods involve a processwith a running time of approximately 6 weeks, and the number of suchgrowing units is limited because of cost reasons, only low yields wereachieved. Moreover, it was not possible by use of previously employedmethods to produce large-volume monocrystals, particularly monocrystalsextending far in all three directions in space, namely preferably roundcrystals with a diameter of >200 mm and a height of >100 mm, becausesuch dimensions regularly lead to block formation within the crystalvolume, namely a reorientation of the crystal axes takes place.Moreover, it has thus far not been possible to obtain satisfactorilysuch large crystals also in optically highly homogeneous form, namely sothat their light refraction is the same in all regions. Another problemwith such crystals is their radiation resistance, namely their abilitynot to undergo discoloration when exposed to a strong radiation source,for example a laser. This problem causes a decrease in yield, forexample in the large-scale production of wafers.

It has already been attempted to produce large monocrystals by growingthem in the shape of plates. EP-A-0 338 411, for example, describes anapparatus and a method for the controlled growing of large monocrystalsin plate shape from a melt and by use of a melting crucible which has arectangular cross-section and is configured so as to present tworelatively wide and two relatively narrow side walls with heatingdevices disposed immediately adjacent to the wide sides. In this case,after the melting, the crucible is slowly lowered from the heatingjacket by means of a lifting device as a result of which the cruciblecontents cool and crystallize. Although by this method it is possible toproduce large oriented monocrystal plates, said plates do not adequatelyextend in all three directions in space.

SUMMARY OF THE INVENTION

Hence, the object of the invention is to produce large-volume crystalswhich are grown in any selected orientation along their {h,k,l} axes,preferably in the {111} or {112} orientation.

Another object of the invention is to produce large-volume crystalswhich extend far in all three directions in space.

According to the invention, this objective is reached by means of themethod and device defined in the claims.

Surprisingly, we have now found that large-volume crystals can beproduced by allowing them to cool with the aid of an axially disposedtemperature gradient, namely a temperature gradient that is parallel tothe growing direction, or with the aid of an axial heat flow whileavoiding a radial, lateral heat flow perpendicular to it. In thismanner, a nearly planar boundary is formed between the solid crystallineand the molten liquid phase. In contrast to this method, by the growingmethods of the prior art, a lateral radial heat flow is predominantlyformed, either as the only heat supply or heat removal or in combinationwith the heating elements disposed above and below a melting crucible.

The device according to the invention for growing large-volumemonocrystals comprises a closable housing, a melting vessel containedtherein and at least one heating element with a heat output sufficientto melt the crystal raw material present in the melting vessel and/or tokeep the already molten crystal raw material in the molten state.

The melting vessel is preferably round. In special cases, however, amelting vessel with an oval or quadrangular cross-section may also beadvantageous. The melting vessel comprises an internal receiving spaceor melting space formed by the bottom of the melting vessel, the sidewalls and an upper opening opposite the bottom.

In a particular embodiment of the invention, the upper opening oppositethe bottom is closed by means of a cover. Said cover is preferablyconfigured so that it does not rest on the side walls in gas-tightfashion, but so that the volatile impurities formed upon melting canleave the melting or crystal space. Laterally, around the meltingcrucible, there is disposed at least one element and preferably severalelements which prevent a radial lateral heat flow. Preferably, thelateral elements are heat insulators, particularly those made ofheat-insulating material. In a particular embodiment, the deviceaccording to the invention has a supporting heating system disposedlaterally at a distance from the melting crucible, which is intended toprevent lateral heat flow. Advantageously, this supporting heatingsystem is disposed at a distance from the melting vessel that issufficient to prevent the generated heat from exerting any directinfluence on the processes taking place in the melt. The supportingheating system only serves to equalize any temperature gradients arisingbetween the melting crucible and the heat flow-preventing elements thatsurround said crucible, and the surroundings. Said supporting heatingsystem is therefore usually separated from the walls of the meltingcrucible by an interposed layer of heat-insulating elements. Preferably,the supporting heating system is configured as a heating jacket.

The bottom of the melting vessel can be configured as desired. Usually,however, it is sloped downward in conical fashion. It thus forms apyramid or preferably a cone, a truncated pyramid or cone beingparticularly preferred.

The bottom of the crucible is preferably provided with a downwardprotruding well which serves to receive a seed crystal of-a desiredorientation. The seed crystal well is preferably disposed in the middleof the bottom, namely at the tip of the cone or pyramid. In a preferredembodiment of the invention, the seed crystal well has, particularly atits lower end, a cooling element. This cooling element is preferably awater-filled cooling element which during the melting of the crystal rawmaterial protects the seed crystal present in the well from prematureincipient or complete melting. In a preferred embodiment of theinvention, the cooling element is heatable.

In a preferred embodiment of the invention, the melting crucible isprovided above the melting space with a widened buffer space whichserves as a funnel for charging the crystal raw material. In particular,however, it serves to equilibrate the heat given off by a cover heaterto ensure that the heat flow produced will act uniformly on the crystalmass and that any local temperature differences arising at the heaterwill be corrected. Preferably, the actual melting space is provided witha heat-conducting cover separating the insulation space from the actualmelting space. Said cover also serves to equilibrate the temperature andconsists of an only slightly heat-insulating material.

The device according to the invention is provided with at least oneheating element disposed above the melting vessel. Thanks to theinsulation element enveloping the melting vessel and preventing alateral heat flow, the heating of this heating element that is disposedabove the melting vessel produces a heat flow which in the meltingvessel runs exclusively axially. A lateral heat flow is prevented by theinsulation elements. Advantageously, this upper heating element is acover heater.

It has been found advantageous to provide a bottom heater at the bottomof the melting vessel in addition to the cover heater. In this manner,an even better axial heat flow or temperature gradient can be obtainedbetween the cover heater and the bottom heater, a heat flow that can beadjusted with unusual sensitivity.

Advantageously, the bottom heater is disposed below the bottom of themelting vessel in a manner such that said heater does not include theseed crystal well or is at least disposed at a distance opposite saidwell in insulating fashion to prevent undesirable premature melting ofthe seed crystal.

In a preferred embodiment of the invention, all heating elements areenclosed in a jacket of insulating material surrounding the meltingvessel, thus preventing an undesirable or uncontrolled heat flow.

It was found advantageous to provide one or more temperature-measuringelements disposed, if possible, closely adjacent to the outer wall ofthe melting vessel. Preferably, the measuring element(s) is (are) in theform of a sliding element(s) which during the operation of the system is(are) slidably disposed along the side wall to ensure measurement of thetemperature gradient extending axially from the bottom of the vessel tothe vessel cover. Preferred measuring elements are thermocouples,thermistors and particularly pyrometers.

In a particularly preferred embodiment, the device of the invention isprovided with an arrangement enabling the boundary between the solidcrystalline phase and the melted liquid phase to be determined. A phasefeeler has been found advantageous for this purpose, said feelercomprising a feeling rod contained in a hollow guide tube extending intothe melting vessel. The rod in the guide tube can be slowly lowered tofeel the solid phase. In another preferred embodiment, the phase feelerconsists of an ultrasonic device which dips into the melt from above andmeasures the sound waves reflected from the phase boundary, indicatingcrystal growth in this manner.

In another embodiment of the invention, the device is provided with acondenser disposed above the opening of the melting crucible, saidcondenser eliminating material vapors that may be escaping. In anotherpreferred embodiment, the housing of the device of the invention isprovided with a cover that can be opened and closed thus allowing thecrystal raw material to be charged to the melting vessel and thefinished crystal to be removed therefrom. Preferably, the housing of thedevice has at least one opening for admitting air to and removing airfrom the entire inner space. Through this opening, the inside of thedevice can be placed under vacuum and/or optionally filled with aprotective gas.

The elements of the device of the invention disposed inside the housingpreferably consist of graphite, the melting vessel being made of highlyheat-conducting, pressed graphite. The insulating material preferablyconsists of loosely packed graphite, particularly fibrous material madeof graphite wool or graphite mats. The heating elements are alsoadvantageously made of graphite. The heat-producing, electricallyconductive graphite strips wind in meander-like fashion around thesurface to be heated and generate heat as electric resistance heaters.To prevent short circuits, the current-conducting elements are kept at adistance from the neighboring graphite parts by means of insulators.Boron nitride insulators are preferred for this purpose.

The housing of the apparatus of the invention usually consists of achemically and heat-resistant material, preferably an alloy steel,high-quality alloy steel being particularly preferred. In many cases,however, structural steel was found to be adequate.

The invention also relates to a method for producing large-volumemonocrystals. According to the invention, said method comprises meltinga crystal raw material mass in a vessel provided with a bottom, sidewalls, an upper opening and optionally a cover which at least partlycloses the upper opening. In principle, it is also possible to introducethe already molten crystal raw material mass into the melting vessel.

According to the invention, to produce large-volume monocrystals, themelt is slowly cooled starting from the crucible bottom and in thedirection of the melt surface or cover heater disposed above the meltsurface. As a result, at the bottom of the vessel is formed a seedcrystal which grows along the temperature gradient or along the heatflow axis. According to the invention, only a single heat flow ortemperature gradient is formed between the bottom of the vessel and themelt surface. This means that, in the melting crucible, temperaturesurfaces facing each other are formed which are planar, the temperatureincreasing from the bottom of the crucible to the surface of the melt orcover heater, and the temperature being the same at all points within atemperature surface or temperature plane, namely not varying by morethan 2° C. Preferably the increase in temperature between the bottom ofthe vessel and the surface of the melt is continuous. Should thesurfaces wherein the temperature is constant exhibit a minimalcurvature, the radius of this curvature is ≧1 m, radii of >2 m andparticularly of >4 m being especially preferred.

In this manner, a phase boundary is formed between the solid,crystalline and the liquid, molten phase, said boundary forming alongthe temperature profile, namely parallel to the planes of equaltemperature, and growing perpendicularly to said planes.

The phase boundary needed for the crystal growing according to theinvention is obtained by preventing the formation of a lateral, namelyradial heat flow. This is achieved with the aid of lateral elements,particularly elements disposed around the walls of the melting vessel.Such elements preferably consist of a supporting heater and/or aninsulating material. It is particularly preferred to dispose along, andat a distance from, the side walls heating elements which serveexclusively to maintain the temperature. In this case, the interspacebetween the melting vessel and the heater disposed at a distancetherefrom is filled with insulating material which on the one handprevents lateral heat removal and thus the formation of a radialtemperature gradient and, on the other, keeps the supporting heater fromcausing local overheating in the melting vessel. In essence, thesupporting heater serves to counteract any heat loss through theinsulating jacket thus supporting the maintenance of radial planarity ofthe temperature profile.

In principle, it is possible to form the axial heat flow formed by themethod of the invention by means of a cover heater disposed above thecrucible. It is preferable, however, also to heat the bottom of thecrucible by means of a bottom heater so as to form a temperaturegradient between the cover heater and the bottom heater.

According to the invention, it is preferred to promote crystal growthwith the aid of a seed crystal placed at the bottom of the meltingvessel. The seed crystal is preferably a monocrystal which is introducedinto a seed crystal well connected to the bottom of the vessel andpreferably so that the orientation of said seed crystal corresponds tothe desired later orientation of the large-volume monocrystal. Themelting is then carried out by turning on the cover heater andpreferably the bottom heater so that the crystal raw material present inthe vessel is melted. Optionally, the jacket heater is also turned on toprovide support. The crucible is thus preferably heated to a temperatureat which possibly present water of crystallization is released first.Thereafter, the temperature is raised further to remove any dissolvedgases or gaseous constituents retained in the crystal raw material mass,as well as any gaseous decomposition products formed during the heat-up.

The melt is then homogenized over a prolonged period, preferably for atleast one day. In particular, this is achieved by adjusting the heatoutput of the heating elements so that convection is generated in themelt which is thus continuously mixed. As a result, dissolved andundesirable impurities reach the melt surface wherefrom highly volatilesubstances, in particular, can vaporize off. Any crystal material thatmay be entrained is collected by means of a, preferably cooled,condenser. The homogenization of the melt is preferably carried out forat least two days and particularly for at least five days, at least oneweek being especially preferred.

During the melting and the homogenization, the seed crystal present inthe seed crystal well is preferably cooled to prevent prematureincipient or complete melting. This is usually accomplished by means ofwater cooling. Advantageously, the cooling is accomplished by means of awater-cooled graphite rod.

At the end of the melting and homogenization of the melt, the seedcrystal is carefully melted. This is normally done in that the coolingis reduced and/or a seed crystal well heater is inserted. The seedcrystal is carefully melted from the top downward so that a uniformtransition arises between the seed crystal and the melt. The axialtemperature gradient is then formed either by slowly lowering the heatoutput of the cover heater and/or of the bottom heater. Preferably,however, the cover heater is set at a temperature which is the same as,or is above or preferably slightly above, the crystallizationtemperature of the crystal to be produced. Advantageous cover heatertemperatures are 200-300° C. above the crystallization temperature. Thetemperature of the bottom heater is advantageously at least 650° C. andpreferably at least 900° C., but during the growing of the crystal islower than the melt temperature. The heat output of the bottom heater isthen slowly reduced. By reducing the bottom heater to a temperaturebelow the crystallization temperature, the melt slowly cools along theaxial temperature gradient, and the phase boundary formed at the planeof the crystallization temperature is slowly displaced in the meltingvessel from the top downward causing the crystal to grow. In principle,it is also possible to reduce the temperature of the cover heater. Inthis case, the temperature is reduced at a rate for which the crystalgrowth occurs at 0.01 to 5 mm/hour, preferably 0.1 to 1 mm/hour andparticularly 0.2 to 0.5 mm/hour. These values are usually achieved usinga cooling rate of 0.001 to 5° C. per hour.

It has been found advantageous if during the growing process thetemperature in the crystallized phase, namely in the grown crystal, isnot lower then a limiting temperature at which a plastic deformation ofthe crystal is still possible. For this reason, the axial temperaturegradient behind the phase boundary should be as flat as possible.

According to the invention, it has been found advantageous to form inthe lower, conical part of the bottom of the melting crucible, namely inthe part forming the bottom between the seed crystal well and the wallof the crucible, a nonplanar phase boundary curved upward toward thecover and having a curvature radius of <1 m, preferably <0.8 m andparticularly <0.5 m.

After the monocrystal has been produced in this manner, it is annealed.As a result, any crystal nonhomogeneities are removed at an elevatedtemperature, namely the crystal defects are repaired at thistemperature. After the growing and annealing of the crystal, the entirelarge-volume monocrystal is slowly cooled to room temperature. Suchcooling is usually carried out over several days to several weeks and,depending on the phase and stage, preferably at a cooling rate fromabout 0.001° C./hour to 15° C./hour, particularly to 10° C./hour andadvantageously to 1° C./hour, with 0.01° C./hour to 8° C./hour andparticularly 3° C./hour being preferred. For the cooling, too, anessentially axial temperature gradient is preferably retained. In thiscase, however, the lateral support heating may optionally be omitted.Such slow cooling affords large-volume crystals which are unusually freeof stress. The cooling rate is preferably controlled by means of severaltemperature sensors disposed in the device of the invention. In thismanner, the temperature course during cooling can also be controlled. Asfor crystal growing, this is advantageously done with the aid of acomputer.

The crystal raw material used for the method of the invention comprises,in particular, raw materials containing in addition to the crystalmaterial also scavengers which during a homogenization phase react withpossibly present impurities to form readily volatile substances.Preferred crystal materials are MgF₂, BaF₂, SrF₂, LiF and NaF, with CaF₂being particularly preferred. The method of the invention affordslarge-volume monocrystals with a diameter of at least 200 mm, preferablyat least 250 mm and particularly at least 300 mm and with a height of atleast 100 mm, preferably 130 mm and particularly at least 140 mm. Theoptical homogeneity attained throughout the entire crystal volume isunusually, high, meaning that the maximum refractive index variationthroughout the crystal volume corresponds to a maximum difference Δn of≦3×10⁻⁶, preferably >2×10⁻⁶ and particularly >1×10⁻⁶, the stressbirefringence SBR being <3 nm/cm and particularly <2 nm/cm andespecially <1 nm/cm.

The method of the invention is preferably carried out in a vacuumbetween 10⁻³ and 10⁻⁶ mbar (corresponding to 10⁻¹ to 10⁻⁴ Pa) andpreferably between 10⁻⁴ and 10⁻⁵ mbar (10⁻² to 10⁻³ Pa). It isparticularly preferred to carry out the method of the invention in anatmosphere of protective gas, particularly in a nonoxidizing atmosphere.To this end, the entire apparatus of the invention is flushed before oralso during the heating with inert gas or an inert gas mixture.

In carrying out the method of the invention, the device of the inventionis preferably mounted in shock-free fashion. During operation, themelting vessel and the heating elements are firmly mounted in anunchangeable, relationship to each other.

The large-volume crystals obtained by the method of the invention areparticularly well suited for the production of optical components forDUV lithography and for the production of wafers covered withphotoresist and thus for the production of electronic devices. Hence,the invention also relates to the use of monocrystals made by the methodof the invention and/or in the device of the invention for theproduction of lenses, prisms, light-conducting rods, optical windows andoptical devices for DUV lithography and particularly for the productionof steppers and excimer lasers and thus also for the production ofintegrated circuits and electronic devices such as computers containingcomputer chips as well as for other electronic devices containingchip-like integrated circuits.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be explained in greater detail by way of thefollowing figures and example.

FIG. 1 is a schematic cross-sectional view through a device of theinvention for growing large-volume crystals,

FIG. 2 is a perspective view of a monocrystal produced with this deviceand having a diameter of 385 mm and a height of about 160 mm, and

FIG. 3 is a perspective view of monocrystal slices obtained from themonocrystal of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the device of the invention comprises a specialdouble-wall housing 10 wherein is disposed melting crucible 20 made ofpressed graphite, which acts as the melting vessel. Melting crucible 20has an inner space 30 (melting and crystallization space) defined bywalls 22, a conical bottom 24 shaped like a truncated cone and by anupper crucible opening 26 which is partly closed by a cover 28. Thebottom 24 is provided with a downwardly extending seed crystal well 32for a seed crystal. A funnel-shaped buffer chamber 40 made of the samematerial as melting crucible 20 is arranged above the melting crucible20 and connected to it, preferably so as to form a single piece. Bufferchamber 40 also has side walls 42, a cover 48 and a bottom 44 which isconnected with side walls 22 of melting crucible 20. A cover heater 50,50′ is arranged above buffet chamber 40 and is located opposite from abottom heater 52. Bottom heater 52 is disposed to the side of the seedcrystal well 32 and is kept at a distance therefrom to prevent thepremature melting of the seed crystal contained therein. The seedcrystal well has a water-cooled graphite rod 70 at its bottom. Lateralheating elements 56 for the intentional melting of the seed crystal arearranged laterally to the water-cooled graphite rod 70 and the seedcrystal well 32. Moreover, laterally to melting crucible 20 is disposeda temperature-measuring element 60 configured as a sliding element,whereby the axial temperature course can be accurately determined. Sidewalls 22 of vessel 20 are completely surrounded by insulating jacket 82.Insulating jacket 82 is located at a distance from a lateral,jacket-like supporting heater 54. Heating elements 50, 50′, 52, 54 and56 and melting crucible 20 with seed crystal well 32 and the bufferchamber 40 disposed above it are surrounded by an insulating material 80consisting of graphite which prevents lateral heat flow and only permitsthe formation of an axial temperature gradient between the cover heater50, 50′ and melting crucible bottom 24 or bottom heater 52.

The device of the invention also has an opening, not shown, for flushingwith an inert gas or for applying vacuum. To determine the size of thegrowing crystal or the position of the phase boundary, there is provideda phase feeler or detector 90 which extends from the outside through thehousing and contains rod 94 slidably disposed in outer tube 92. This rod94 is capable of being slowly moved downward and of feeling or detectingthe solid phase boundary. In principle, tube 92 can also be connectedwith rod 94. To separate the impurities removed during homogenization,condenser 95 is disposed above the melting crucible.

EXAMPLE 1

To produce a calcium fluoride monocrystal, inner space 30 of meltingvessel 20 was filled with a calcium fluoride mixture, which in additioncontained scavenger materials with high affinity for oxygen, such asPbF₂, SnF₂ or CdF₂. The device was then closed with a cover not shown inFIG. 1 and was flushed with nitrogen as inert gas to remove theundesirable atmospheric oxygen. A vacuum of 10⁻⁴ mbar (10⁻²Pa) was thenapplied and, while cooling the seed crystal well 32, cover heater 50,50′, bottom heater 52 and optionally jacket heater 54 were put in placeand slowly heated to 1450° C. over a period of several hours. The meltwas then homogenized at this temperature for five days while maintainingconvection in it. After the homogenization, the bottom heatertemperature was reduced to a temperature of 1200° C., and the seedcrystal was carefully melted with the aid of seed crystal well heater56. After turning off the seed crystal well heater, cover heater 50, 50′was kept constant while the bottom heater 52 was made to cool slowly,over a period of several days, to a temperature below thecrystallization point, causing a monocrystal to grow from the seedcrystal in the direction of the melt surface. The resulting monocrystalhad the same orientation as the seed crystal. After the entire melt hadsolidified with formation of a monocrystal, the resulting monocrystalwas annealed and cooled to room temperature over a period of threeweeks. In this manner, a monocrystal MC was obtained as shown in FIG. 2,which had a diameter, D, of 385 mm and a height, H, of 161 mm (withoutseed crystal well portion). The monocrystal had throughout the entirecrystal volume a refractive index variation Δn<1×10⁻⁶. The stressbirefringence SBR was <1 nm/cm. Moreover, the crystal thus obtained hadan unusually high radiation resistance.

Monocrystal slices S1, S2, S3, etc, from the monocrystal MC are shown inFIG. 3. A floppy disk FP is also shown in FIG. 3 to provide a feelingfor the size of the monocrystal slices.

1. A large-volume monocrystal having a diameter of at least 300 mm and aheight of at least 140 mm, a maximum refractive index variation througha crystal volume less than or equal to 2×10⁻⁶ and a stress birefringenceSBR than 2 nm/cm; wherein said large-volume monocrystal is made by amethod comprising the steps of: a) providing a device comprising ahousing (10) with a closable charging and discharging opening, a meltingvessel (20) disposed in said housing (10) and enclosing a melting space(30), said melting space (30) being bounded by a bottom (24), side walls(22), an upper opening (26) opposite the bottom and optionally by acover (28) for said upper opening, at least one heating element (50,50′) for melting a crystal raw material mass placed in the melting space(30) and/or for maintaining a molten state of an already melted crystalraw material mass, said at least one heating element (50, 50′) beingdisposed above said melting vessel (20) and generating an axial heatflow extending axially between the bottom (24) and the opening (26) ofthe melting vessel, and at least one element (82, 54) disposed at theside walls (22) of the melting vessel (20) so as to prevent a lateralheat flow; b) heating crystal raw material present in said meltingvessel (20) by said at least one heating element (50, 50′) orintroducing already melted crystal raw material at a temperature above amelting point of the crystal raw material to form a melt with a meltsurface in the melting vessel; c) subsequently lowering the temperatureto at least a crystallization point of the crystal raw material so as toform said monocrystal on the bottom (24) of the melting vessel (20),said monocrystal forming a solid/liquid phase boundary at which saidmonocrystal grows toward the melt surface in a direction perpendicularto said phase boundary; and d) maintaining a vertical axial temperaturegradient between the bottom (24) of the melting vessel (20) and theupper opening (26) and preventing heat inflow and/or heat outflowthrough said side walls (22) by means of the at least one element (82,54), so that said solid/liquid phase boundary has a curvature radius ofat least one meter.
 2. The large volume monocrystal as defined in claim1, wherein said maximum refractive index variation through the crystalvolume is less than or equal to 1×10⁻⁶ and said stress birefringence SBRis less than 1 nm/cm.
 3. The large volume monocrystal as defined inclaim 1, wherein said crystal raw material is a metal fluoride selectedfrom the group consisting of calcium fluoride, magnesium fluoride,barium fluoride, strontium fluoride, lithium fluoride and sodiumfluoride.
 4. A method for growing a large-volume monocrystal with auniform orientation from a melt of crystal starting material, saidmethod comprising the steps of: a) providing a device for growing thelarge-volume monocrystal comprising a housing (10) with a closablecharging and discharging opening, a melting vessel (20) disposed in saidhousing (10) and enclosing a melting space (30), said melting space (30)being bounded by a bottom (24), side walls (22), an upper opening (26)opposite the bottom and optionally by a cover (28) for said upperopening, at least one heating element (50, 50′) for melting a crystalraw material mass placed in the melting space (30) and/or formaintaining a molten state of an already melted crystal raw materialmass, said at least one heating element (50, 50′) being disposed abovesaid melting vessel (20) and generating an axial heat flow extendingaxially between the bottom (24) and the opening (26) of the meltingvessel, and at least one element (82, 54) disposed at the side walls(22) of the melting vessel (20) so as to prevent a lateral heat flow; b)heating crystal raw material present in said melting vessel (20) by saidat least one heating element (50, 50′) or introducing already meltedcrystal raw material at a temperature above a melting point of thecrystal raw material to form a melt with a melt surface in the meltingvessel; c) subsequently lowering the temperature to at least acrystallization point of the crystal raw material so as to form saidmonocrystal on the bottom (24) of the melting vessel (20), saidmonocrystal forming a solid/liquid phase boundary at which saidmonocrystal grows toward the melt surface in a direction perpendicularto said phase boundary; and d) maintaining a vertical axial temperaturegradient between the bottom (24) of the melting vessel (20) and theupper opening (26) and preventing heat inflow and/or heat outflowthrough said side walls (22) by means of the at least one element (82,54), so that said solid/liquid phase boundary has a curvature radius ofat least one meter.
 5. The method as defined in claim 4, wherein saiddirection of growth of the monocrystal is set in advance by means of aseed crystal.
 6. The method as defined in claim 4, wherein saiddirection of growth of the monocrystal corresponds to a {111} or {112}crystal axis.
 7. The method as defined in claim 4, wherein said verticalaxial temperature gradient is maintained by means of a bottom heater(52) arranged below said bottom (24) of said melting vessel (20) and acover heater (50) arranged above said upper opening (26).
 8. The methodas defined in claim 7, wherein a crystal growth rate of 0.1 to 1 mm/hris produced by slowly reducing an amount of heat produced by the bottomheater (52).
 9. The method as defined in claim 7, wherein said bottomheater (52) is maintained at a temperature, which is at the most 10° C.below the melting point of the crystal raw material.
 10. The method asdefined in claim 4, further comprising cooling said melt and/or saidmonocrystal at a cooling rate of 0.1 to 5° C. per day.
 11. The method andefined in claim 4, further comprising, after introducing said crystalraw material into said melting vessel (20), flushing said deviceincluding a remaining portion of the melting vessel (20) with an inertgas and/or growing said monocrystal under vacuum.
 12. The method asdefined in claim 4, wherein said crystal raw material is calciumfluoride.
 13. The method as defined in claim 4, further comprisingadding one or more scavengers to said crystal raw material.
 14. Themethod as defined in claim 4, wherein said lateral heat flow isprevented by means of a supporting heater (54) kept at a distance fromthe melt vessel walls by means of a heat insulator (82).
 15. The methodas defined in claim 4, further comprising heating the bottom heater (61)to a temperature of at least 650° C. and the cover heater to atemperature of at least 1590° C. at a beginning of monocrystal growth.16. The method as defined in claim 4, further comprising homogenizingsaid melt over a time period of 1 to 20 days prior to a start ofmonocrystal growth.
 17. The method as defined in claim 7, furthercomprising maintaining said cover heater at a constant temperature whilesaid bottom heater (52) is slowly cooled.
 18. A device for growing alarge-volume monocrystal, said device comprising a housing (10) with aclosable charging and discharging opening; a melting vessel (20)disposed in said housing (10) and enclosing a melting space (30), saidmelting space (30) being defined by side walls (22), a bottom (24), anupper opening (26) opposite the bottom and an optional cover (28) forsaid upper opening; at least one heating element (50, 50′) for melting acrystal raw material mass placed in the melting space (30) and/or formaintaining a molten state of an already melted crystal raw materialmass, said at least one heating element (50, 50′) being disposed abovesaid melting vessel (20) and generating an axial heat flow extendingaxially between the bottom (24) and the opening (26) of the meltingvessel; and at least one element (82, 54) disposed at the side walls(22) of the melting vessel (20) so as to prevent a lateral heat flow.19. The device as define in claim 18, wherein said at least one element(82, 54) disposed at the side walls (22) prevents formation of saidlateral heat flow to a sufficient extent so that during crystal growth aplanar solid/liquid phase boundary with a curvature radius of ≧1 m isformed.
 20. The device as defined in claim 18, wherein said bottom (24)of the melting vessel (20) has a conical shape.
 21. The device asdefined in claim 18, wherein said bottom (24) of the melting vessel (20)is provided with a seed crystal well (32) for receiving a seed crystal.22. The device as defined in claim 21, further comprising a coolingelement (70) disposed at said seed crystal well (32).
 23. The device asdefined in claim 18, further comprising a temperature-measuring element(60) is provided at the side wall (22) of the melting vessel (20). 24.The device as defined in claim 19, further comprising an element (90)for detecting said solid/liquid phase boundary.
 25. The device asdefined in claim 18, wherein said heating elements, the at least oneelement for preventing lateral heat flow and/or said melting vessel aremade of graphite.
 26. The device as defined in claim 18, wherein said atleast one element (82, 54) comprises a supporting heater (54) spaced adistance from the side walls (22) of the melting vessel.
 27. The deviceas defined in claim 18, further comprising a bottom heater (52) arrangedbelow said bottom (24) of said melting vessel (20), said bottom heatercomprising means for maintaining a vertical axial temperature gradientin said melt.